A healthy heart is normally caused to contract and relax in an orderly fashion by a spreading wave of electrical excitation originating from the sinoatrial (SA) node in the right upper atrium. The wave initiated at the SA node spreads over cardiac fibers until it reaches the atrioventricular (AV) node. The AV node then relays the wave over specialized cardiac fibers known as the bundle of His, to the ventricles. The cardiac fibers over which the impulses are conducted have a refractory period, so that once stimulated they cannot be restimulated for a short time period. This normally serves as a protective mechanism.
However some people are born with an accessory pathway of cardiac fibers extending from the ventricle near the area of the AV node back to the atrium. The accessory pathway allows the excitation wave from the AV node to retrograde or travel back to the atrium.
In some cases, if the retrograde wave reaches the atrium just after the end of a refractory period, it can then travel back to the AV node, stimulating the AV node prematurely and producing an oscillatory loop. Various other mechanisms, e.g. partial damage to atrial or ventricular heart muscle, can also result in an oscillatory loop. The oscillatory loop causes abnormally rapid heart action (tachycardia). This is usually self limiting, but in cases where it is not, it may be fatal. Therefore the condition requires treatment.
Tachycardia and other arrhythmias have sometimes been treated with medication. However the medication is not always effective and may have serious side effects.
A second method of treating the condition has been open heart surgery, to cut the tissue (e.g. the accessory pathway) which forms part of the feedback loop, thus opening the feedback loop. However open heart surgery is a serious and costly operation.
Therefore, for about the last fifteen years cardiologists have attempted to deal with the condition by inserting catheters containing electrodes into the interior of the heart. They have attempted to locate the accessory pathway or other tissue in question and then to apply RF energy to ablate the tissue by coagulation. The catheters are pushable and steerable, and are guided to the approximate location by x-rays for general guidance, and then by the use of electrograms to the exact location for fine localization. For example, the fibers known as the bundle of His, emanating from the AV node, are close to the accessory pathway, so cardiologists often look for electrograms with His activity to determine that the catheter is close to the accessory pathway.
Numerous catheters have been designed to perform the above functions. Examples are shown in U.S. Pat. No. 5,242,441 to Avitall, U.S. Pat. No. 5,125,896 to Hojeibane, and U.S. Pat. No. 5,190,050 to Nitzsche. Various designs compete on the basis of which is more easily steerable.
In an article by L. T. Blouin and F. I. Marcus, in Pace, Vol. 12, January, 1989, part 2, pages 136 to 143, it was disclosed that by insulating part of the tip of the catheter, and applying the bare (conductive) portion of the tip against dog ventricular muscle in vitro, larger lesions could be produced with lower RF power. However so far as is known, this technique has not been used to date in any practical catheters in animal or human subjects.
Regardless of which catheter is used to date, a serious problem has existed and currently exists in determining whether the catheter is in the right location to form an ablation. It is common for the surgeon to guide an ablation catheter to a location which is believed to be correct, then to apply RF power to create an ablation, and then to observe that nothing has happened. In that case the surgeon then moves the catheter elsewhere and tries again. In many of these cases, the catheter may in fact have been properly positioned, but because most of the tip of the catheter is bathed in blood rather than lying against the tissue, and since blood has a lower electrical impedance than the tissue, the RF power has effectively been short circuited through the blood and an adequate ablation has not been formed. Problems of this kind can greatly increase the time required for catheterization procedures and can decrease the likelihood of a successful result.